20 THE HUMAN BOD I. 



pounds into simpler ones, sometimes called dissimilation, 

 is as invariable in living beings as the building up of new 

 complex molecules referred to above. It is associated with 

 the assumption of uncombined oxygen from the exterior, 

 which is then combined directly or indirectly with 

 other elements in the cell, as for example carbon, 

 giving rise to carbon dioxide, or hydrogen producing 

 water. In this w r ay the molecule in which the carbon and 

 hydrogen previously existed is broken down, and at the 

 same time energy is liberated, which in all cases seems to 

 take in part the form of heat just as when coal is burnt 

 in a fire, but maybe used in part for other purposes such as 

 producing movements. The carbon dioxide is usually got 

 rid of by the same mechanism as that which serves to take 

 up the oxygen, and these two processes constitute the 

 function of respiration which occurs in all living things. 

 Assimilation and disassimilation, going on side by side and 

 being to a certain extent correlative, are often spoken of 

 together as the process of nutrition, a term which there- 

 fore includes all the chemical transformations occurring in 

 living matter. 



Contractility. Nutrition and (with the above-mentioned 

 partial exception) reproduction characterize all living crea- 

 tures; and both faculties are possessed by the simple 

 nucleated cells already referred to as found in our blood. 

 But these cells possess also certain other properties which, 

 although not so absolutely diagnostic, are yet very charac- 

 teristic of living things. 



Examined carefully with a microscope in a fresh-drawn 

 drop of blood, they exhibit changes of form independent 

 of any pressure which might distort them or otherwise 

 mechanically alter their shape. These changes may some- 

 times show themselves as constrictions ultimately leading 

 to the division of the cell; but more commonly(Fig. 12*) 

 they have no such result, the cell simply altering its form 

 by drawing in its substance at one point and thrusting it 

 out at another. The portion thus protruded may in turn 

 be drawn in and a process be thrown out elsewhere ; or the 

 rest of the cell may collect around it, and a fresh protru- 



*P. 48. 



